Reducing the number of elevator shafts used in a building has been attempted for years in order to increase the space available for profitable purposes. It is expected that the number of hoistways in some large buildings could be reduced by over 80%. For years, double deck cabs have been used in order to increase the passenger capacity of a hoistway, with each deck serving even or odd floors. However, double deck cabs can limit the freedom of travel provided to passengers. Some systems have used multiple one-way shafts with several cabs being transferred from one shaft to another in order to create a loop of travel. This has proven to be too costly due to complexity and increased energy usage.
Another way to accomplish this is by having multiple cabs in a single shaft. The number of cabs in an individual shaft has been limited to two or three cabs due to the auxiliary equipment used for operation of one cab interfering with another cab's operation. Placing counterweights for the various cabs that do not interfere with each other can be a significant problem as the number of cabs increases. Using one central counterweight or two counterweights on opposing corners of a cab can result in less than ideal balance of the cabs. In some instances, the space needed for the travel of counterweights can be reduced, but this may require cable storage outside of the existing footprint of the elevator shaft. This is a drawback, since a significant advantage of a multiple cab elevator system is reducing the footprint used.
Elevator systems capable of using multiple cabs are also usually incapable of operating in an existing elevator shaft without substantial modification. This can significantly increase the installation cost of such a system.